According to, a double pane window with an emissivity = 0.05 coating and a 1/4 air gap has a measured (center of glass) U-factor

According to, a double pane window with an emissivity ε = 0.05 coating and a 1/4" air gap has a measured (center of glass) U-factor of 2.33W/m² K. Assume that this coating is sufficient to stop all radiative heat transfer, so that the thermal resistance of this window comes from conduction in the glass, the air gap, and the still air layers on both sides of the window. Taking R_{still air} ≈ 0.17m² K/W from Example 6.1, compute the U-factor of this window ignoring convection in the air gap. Use the same method to compute the U-factor of a similar window with a 1/4" argon-filled gap (measured value U = 1.87W/m² K). Repeat your calculation for a 1/2" air gap (measured U factor l.70 W/m² K). Can you explain why your answer agrees less well with the measured value in this case?